The present invention relates to measurement pertaining to fluids, more particularly to methods and apparatuses for effecting measurement of the velocity or concentration of fluids using molecular tracing material.
According to the flow velocity measurement technique known as particle-image velocimetry (PIV), small seed particles are used to track the flow of a fluid. The particle seeding required for particle-image velocimetry can be undesirable or lead to inaccurate results in some situations. For example, seeding the flow medium at the density required to resolve the finest relevant length scales in complex flows may itself influence the flow dynamics. This can be the case in very-high Reynolds number turbulence (such as boundary layers on ships or submarines), or in micro-fluidic devices (wherein typical dimensions can approach the diameter of seed particles). In other situations, velocimetry techniques that rely upon particles can be inaccurate due to flow-tracking problems. These can arise when the fluid is strongly accelerated, and seed particles are density-mismatched with the fluid. See Adrian, R. J., “Particle-Imaging Techniques for Experimental Fluid Mechanics,” Annual Reviews in Fluid Mechanics, vol. 23, 1991, pages 261–304, incorporated herein by reference. Heavy particles, for instance, can lag the fluid flow in shocks, and may be centrifuged out of vortex cores. Other flow tracking problems may arise in the presence of large temperature or electric-potential gradients (as in electro-hydrodynamic flows) due to thermophoretic and electrophoretic forces acting on the particles.
Molecular-tagging velocimetry, also known as laser-induced photochemical anemometry, is a flow velocity measurement technique that uses luminescent molecular tracers instead of particles to track the motion of moving fluids. See, e.g., the following references, each incorporated herein by reference: Gendrich, C. P., Koochesfahani, M. M. and Nocera, D. G., “Molecular Tagging Velocimetry and Other Novel Applications of a New Phosphorescent Supramolecule,” Experiments in Fluids, volume 23, pages 361–372, 1997; Falco, Robert E. and Nocera, Daniel G., “Quantitative Multipoint Measurement and Visualization of Dense Solid-Liquid Flows by Using Laser Induced Photochemical Anemometry (LIPA),” Particulate Two-Phase Flow, M. C. Rocco, Ed., Butterworth-Heinemann, Boston, 1993, chapter 3, pages 59–126. Flow measurement using molecular tracers is desirable when techniques that rely upon seeding the fluid with small particles are unusable or deficient.
Molecular-tagging velocimetry uses molecular tracers rather than seed particles, thus representing a viable alternative to particle-image velocimetry. According to molecular-tagging velocimetry, flow tracers such as caged-fluorescent molecules or long-lifetime phosphorescent compounds are uniformly mixed with the flow medium and selectively excited with illumination at the appropriate wavelength. The excited regions, which luminesce, are imaged at two successive times. Lagrangian velocities in the flow are estimated from the displacement of the selectively excited regions. An important aspect of molecular-tagging velocimetry is its ability to selectively illuminate regions in the flow. This has been done in the past by illuminating single or multiple planes in the flow medium.
In order to measure two components of velocity in the practice of molecular-tagging velocimetry, the selective excitation of the flow medium must generate gradients in at least two (preferably orthogonal) directions. This has been done via the splitting and expanding of a laser beam into two sheets, and the blocking of each sheet with a comb-like beam blocker, such as shown in FIG. 5 of the aforementioned Gendrich et al. reference. Gendrich et al's FIG. 5 is a schematic illustrating a conventional illumination system for flow measurements. The region “tagged” by a grid pattern using beam blockers in this manner as disclosed by Gendrich et al. is shown in their FIG. 5 to be characterized by (approximate) uniformity of slot width and spacing. Examples of conventional beam-blockers are shown in Gendrich et al.'s FIGS. 4a and 4b. Gendrich et al.'s FIG. 4a shows 2 mm front-silvered mirror slivers glued to a steel substrate. Gendrich et al.'s FIG. 4b shows alternating 1.6 mm and 0.8 mm slots cut into 0.75 mm thick aluminium. The resulting grid pattern in the fluid can be used to make velocity measurements in two-dimensional plane.
A summary of illumination methods for molecular-tagging velocimetry is given by Koochesfahani, M. M., “Molecular Tagging Velocimetry (MTV): Progress and Applications,” AIAA Paper AIAA-99–3786, invited, American Institute of Aeronautics and Astronautics, 30th AIAA Fluid Dynamics Conference, Norfolk, Va., 28 Jun.–1 Jul., 1999, incorporated herein by reference. Also of interest is Koochesfahani, Manochehr M. and Nocera, Daniel G., “Molecular Tagging Velocimetry Maps Fluid Flows,” Laser Focus World, Jun. 2001, pages 103–108, incorporated herein by reference.
Existing flow-illumination procedures for practicing molecular-tagging velocimetry have several limitations, including the following. First, according to these existing techniques, the illumination ideally should come from a direction perpendicular to the viewing direction; this requires optical access to the flow from two different directions (one for viewing, one for illumination), which can be difficult to arrange in experiments. Moreover, the illumination pattern is limited to simple grids. Furthermore, diffraction from the beam blockers limits the minimum line width and spacing for the grid pattern; this limits the resolution of the flow measurement.